In order to enable the liquid in a container to be uniformly aerated with a gas, one usually aims at achieving a uniform distribution of the rotor-impelled liquid-gas mixture over the bottom of the container, in order to ensure a likewise uniform distribution of the rising fine, i.e., very small, gas bubbles over the cross-section of the container. Ordinarily, however, the stator is far smaller in diameter than the container, and thus the cross-sectional area of the aeration region, i.e., the generally columnar region of the body of liquid through which the gas or air bubbles rise toward the surface of the liquid and the perimeter of which region lies at most only a relatively small distance radially outwardly of the projection of the periphery of the stator onto the container bottom, extends through only a relatively small portion of the body of liquid to be aerated. For example, in a large container such as a tank or basin 10 m in diameter (or 10.times.10 m in size if square) or a lagoon which can be 50.times.100 m in size or even greater, the aeration region is about 4 m in diameter at most, depending on the size of the aerator. A large container of this type, therefore, cannot be totally aerated by the aerator alone because the bubbles leaving the rotor cannot spread over the entire expanse of the container bottom before rising to the surface.
Quite to the contrary, in a large container the air bubbles rise initially uniformly through a generally columnar region above the centrifugation zone of the submersible aerator, which region, as mentioned, depending on the size of the aerator, is approximately 4 m in diameter. The work generated by the expansion of the air bubbles drives the liquid upwardly. As the level of the liquid above this region is elevated somewhat, the elevated liquid flows at first radially outwardly and then, after a certain outward flow, begins to flow back downwardly until, when near the floor of the basin, it flows back toward the center of the aeration region. As a result of this circulating flow, the descending liquid throttles the air emission from the aerator. This causes the rising air, and with it the liquid, to be confined to a somewhat smaller cross-section, although the quantity of displaced liquid remains the same since it depends only on the work output of the rising quantity of air. With a normal liquid head of about 4 m above the container bottom, therefore, the velocity of upward flow of the liquid attains values which lie between 0.2 and 0.5 m/s. The gas bubbles, however, rise about 0.2 m/s faster than the liquid and thus reach the upper surface of the liquid in a very short time, for example, within 6 to 10 seconds. That means that the residence time of the air bubbles in the liquid is relatively small (as it would be, for example, in a body of liquid only about 1.2 to 2 m deep), with the result that due to the liquid circulation, which is also known as the "airlift effect," the oxygen transfer efficiency (the oxygen consumption expressed in %) and therewith the standard oxygen transfer rate (the oxygen uptake expressed in kg O.sub.2 /h), as those terms have been defined by the ASME, are correspondingly lower than they should be.
To overcome this drawback, and expressly for the purpose of enabling the basal area of the aeration region at the bottom of the container to be enlarged, it is known from EP-A-204 688 to construct the stator surrounding the rotor as a closed ring of circumferentially distributed, non-radially extending flow channels, by means of which a correspondingly higher outflow velocity of the liquid-gas mixture and thereby also a larger area of centrifugation and expulsion of the liquid-gas mixture can be achieved. The described arrangement, with a suitable limit being imposed on the angular spacing between the flow channels by virtue of the provision of a sufficient number of flow channels, enables a uniform aeration to be achieved over a region having a larger basal area.
The same purpose is served by another known apparatus, disclosed in FR-B-2 444 494, in which the stator is constructed as a mixing chamber for the air directed to the rotor vanes and the liquid drawn into the aerator. In this arrangement, a plurality of distributing pipes for the liquid-gas mixture communicate with the mixing chamber, which again is intended to enable a larger area of centrifugation and expulsion of the liquid-gas mixture to be achieved.
In these known systems, however, it is disadvantageous that the lengths of the distributing pipes cannot be enlarged as desired, because as the pipe length increases the risk exists that the initially created fine small gas bubbles of the liquid-gas mixture will combine and merge with each other in the distributing pipes into the form of larger gas bubbles. This would lead to the result that the sought-for fine distribution of small gas bubbles over the basal area of the provided aeration region is not achieved. In order to avoid having the small gas bubbles exiting from the mixing chamber into the adjacent distributing pipes merge with each other into the form of larger bubbles, it is known from DE-C-3 210 473 to utilize distributing pipes of different lengths, but the maximum attainable expulsion area remains restricted because the different distributing pipe lengths at most have an effect on the gas bubble distribution within the provided expulsion area.